1. Field of the Invention
The present invention relates to an image forming apparatus, in particular, to that capable of forming an image on a printing medium by causing toner to soar and adhere to the printing medium from a toner bearing member via toner passage holes which are controlled to be open and close.
2. Discussion of the Background Art
As a conventional image forming apparatus, a toner jet, direct toning, and toner projection system or the like of that directly prints an image on a printing medium or an inter mediate transfer medium with toner is well known.
For example, the Japanese Patent Application Laid Open No. 63-136058 discloses a technology in that electric charge generated by friction caused between a fixed blade and a rotation roller is provided to toner, which is supplied from a toner hopper. The toner is then rotationally conveyed and is controlled to soar by a control pulse applied to a control member in an electric field that is created by the rotation roller.
The toner with electric charge electro-statically adheres to the surface of the rotation roller, and thus, needs to be separated by the control pulse. Since there exists a gap of more than 100 micrometer between the rotation roller and the control member, the control pulse necessarily needs a high voltage of more than 500V to execute the separation. Thus, a driver for control use needed corresponding to a number of pixels becomes extraordinary costly. Further, a responsibility becomes deteriorated and delayed due to necessity of separating and causing the toner to soar from the rotation roller.
The Japanese Patent Registration No. 2,933,930 and the Japanese Patent Publication No. 2-52, 260 disclose a technology in that a control pulse is inputted to a control electrode where developer passes while applying an alternating bias between a rotation developer bearing member and a control device.
With this system, the above-mentioned responsibility problem is decreased. However, since an alternating electric field is uniformly entirely provided to a toner soaring region, the developer repeatedly adheres and soars from and to the developer bearing member. Thus, the alternating bias need be intensive to separate the developer adhering to the developer bearing member. Thus, a lot of separated toner swiftly soars to the control device and unavoidably adheres to the electrode of the control device, and thereby reliability decreases. Further, the high voltage of more than 500V need be applied between the developer bearing member and the control device, because a gap exists therebetween. Thus, the control pulse that causes the electric field either to allow or prohibit passage of toner in the electric field similarly needs a high voltage. Thus, the driver cost problem is yet unsolved.
The Japanese Patent Application Laid Open No. 59-181370 discloses that a developer bearing member includes plural electrodes and creates electric fields that change between the plural electrodes as time elapses, so that toner soars toward a control electrode.
Since the toner soaring and floating in the vicinity of the control electrode is controlled, the problem of increase of the control voltage existing in the above-mentioned Japanese Patent Application Laid Open No. 63-136058, the Japanese Patent Registration No. 2933930, and the Japanese Patent Publication No. 2-52260 is resolved.
As in the Japanese Patent Application Laid Open No. 59-181370, the Japanese Patent Application Laid Open No. 02-226261 discloses a technology in that a developer bearing member includes plural electrodes and creates electric fields that change as time elapses between these plural electrodes, so that toner soars. However, a control electrode that controls passage of the toner conventionally arranged on the printing medium side is arranged on a toner supplying surface side.
In this system, the control voltage can be decreased from the conventional 400V down to 100V. Further, when toner adhering to a printing head where the control electrodes are arranged is removed, the toner can be collected in a toner supply source.
For example, as shown in FIG. 23, a conventional direct printing system includes a toner bearing roller 1501 arranged as an agent bearer with its axis being extended left and right in the drawing and bears toner T having been charged thereon while driven rotated by a drive device, not shown. Below the toner bearing roller 1501, a flexible print baseboard (FPC) 1503 is arranged as a hole formation member having plural holes 1502. The FPC 1503 includes plural ring state soar electrodes 1504 surrounding the plural holes 1502 respectively while opposing the toner bearing roller 1501.
Below the FPC 1503, an opposing electrode 1506 is arranged opposing the toner bearing roller 1501. Also arranged is a printing sheet 507 conveyed above the opposing electrode 1506 by a conveyance device. Even only one hole 1502 and such a soar electrode 1504 are illustrated there, these plural combinations are practically formed on the FPC 1503. Specifically, a FPC 1503 for 600 dpi use includes 4960 items of these combinations.
The toner bearing roller 1501 is grounded, for example, and bears toner T charged in a negative polarity. When a soar voltage of the positive polarity is applied to the soar electrode 1504, the toner T on the toner bearing roller 1501 opposing the soar electrode 1504 or that in the vicinity thereof are subjected to an electric field having a prescribed intensity. Due to influence of the electric field, an electrostatic force applied to the toner T exceeds an attraction force attracting the toner T to the toner bearing roller 1501. Thus, aggregation of toner T selectively soars and enters the hole 1502 in a dot state from the loner bearing roller 1501.
Then, the toner T is drawn by an electric field created between the soar electrode 1504 and the above-mentioned intensively charged opposing electrode 1506 and keeps soaring and attracts the toner to the surface of the printing sheet 1507 via the hole 1502, so that the aggregation of the toner T becomes a dot image.
In such a situation, a soar voltage applied to each of the soar electrodes 1504 needs to be controlled to turn on and off independently by a different private use IC. Specifically, when the voltage is high, the image forming apparatus of the direct printing system needs the same number of expensive ICs as the soar electrodes 1504. For example, the FPC 3 for 600 dpi use is employed, 4960 items of expensive switching elements are needed. In general, as voltage endurance increases, an IC becomes expensive due to increase of a chip area. Thus, it is significant for the image forming apparatus of the direct printing system to decrease the control voltage in view of cost.
However, an attraction force attracting each other is created between the toner T and the toner bearing roller 1501 by a mirror image force, a van der Waals force, a liquid cross-link force or the like, and makes the soar voltage difficult to decrease. As a result, the soar voltage of more than 500 v needs to be applied in the above-mentioned apparatus.
However, the voltage applied to the soar electrode can be decreased if the developer bearer includes plural electrodes and a timely changing electric field is created between these electrodes so that toner is made in a cloud state and soars toward the control electrode as described in the Japanese Patent Application Laid Open No. 59-181370.
However, since the toner bearing roller bears the toner with charge on its surface while being driven rotated by a drive device, it necessarily has a prescribed curvature on its surface. Since the control electrode is substantially planer, a space distance between the toner bearing roller and the control electrode is different per line of the electrodes. Owing to this, it is revealed that an amount of the toner passing through a hole on a line of the electrode far distanced from the toner bearing roller is less than that of the other line distanced closer. Further, such tendency becomes prominent in a low voltage applied region. In any event, an amount of toner passing through a toner passage hole need to be highly precisely controlled to achieve high speed and high quality image formation.